A flawed premise, and a little logic, and you can justify giving a student 50% for nothing. Ryan finds it troublesome, and so do I.
However, there's a deeper question: who originally decided that, when it comes to a passing grade, 60% is good enough?
(By the way, success for a National Board candidate is 68.75 percent. I have exactly one week to get the first part of my D+ or better.)